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structboost_1_1hana_1_1optional.html

structboost_1_1hana_1_1optional.html 104 KB
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  136. <div class="title">boost::hana::optional&lt; T &gt; Struct Template Reference<div class="ingroups"><a class="el" href="group__group-datatypes.html">Data types</a></div></div>  </div>
    137. 

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  139. <a name="details" id="details"></a><h2 class="groupheader">Description</h2>
    140. 

  140. <div class="textblock"><h3>template&lt;typename ... T&gt;<br />
    141. 

  141. struct boost::hana::optional&lt; T &gt;</h3>
    142. 

  142. 

  143. <p>Optional value whose optional-ness is known at compile-time. </p>
    144. 

  144. <p>An <code>optional</code> either contains a value (represented as <code>just(x)</code>), or it is empty (represented as <code>nothing</code>). In essence, <code><a class="el" href="structboost_1_1hana_1_1optional.html" title="Optional value whose optional-ness is known at compile-time.">hana::optional</a></code> is pretty much like a <code>boost::optional</code> or the upcoming <code>std::optional</code>, except for the fact that whether a <code><a class="el" href="structboost_1_1hana_1_1optional.html" title="Optional value whose optional-ness is known at compile-time.">hana::optional</a></code> is empty or not is known at compile-time. This can be particularly useful for returning from a function that might fail, but whose reason for failing is not important. Of course, whether the function will fail has to be known at compile-time.</p>
    145. 

  145. <p>This is really an important difference between <code><a class="el" href="structboost_1_1hana_1_1optional.html" title="Optional value whose optional-ness is known at compile-time.">hana::optional</a></code> and <code>std::optional</code>. Unlike <code>std::optional&lt;T&gt;{}</code> and <code>std::optional&lt;T&gt;{x}</code> who share the same type (<code>std::optional&lt;T&gt;</code>), <code>hana::just(x)</code> and <code>hana::nothing</code> do not share the same type, since the state of the optional has to be known at compile-time. Hence, whether a <code>hana::just</code> or a <code>hana::nothing</code> will be returned from a function has to be known at compile-time for the return type of that function to be computable by the compiler. This makes <code><a class="el" href="structboost_1_1hana_1_1optional.html" title="Optional value whose optional-ness is known at compile-time.">hana::optional</a></code> well suited for static metaprogramming tasks, but very poor for anything dynamic.</p>
    146. 

  146. <dl class="section note"><dt>Note</dt><dd>When you use a container, remember not to make assumptions about its representation, unless the documentation gives you those guarantees. More details <a class="el" href="index.html#tutorial-containers-types">in the tutorial</a>.</dd></dl>
    147. 

  147. <h2>Interoperation with <code>type</code>s </h2>
    148. 

  148. <p>When a <code>just</code> contains an object of type <code>T</code> which is a <code>type</code>, it has a nested <code>::type</code> alias equivalent to <code>T::type</code>. <code>nothing</code>, however, never has a nested <code>::type</code> alias. If <code>t</code> is a <code>type</code>, this allows <code>decltype(just(t))</code> to be seen as a nullary metafunction equivalent to <code>decltype(t)</code>. Along with the <code>sfinae</code> function, this allows <code><a class="el" href="structboost_1_1hana_1_1optional.html" title="Optional value whose optional-ness is known at compile-time.">hana::optional</a></code> to interact seamlessly with SFINAE-friendly metafunctions. Example: </p><div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="not_8hpp.html">boost/hana/not.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="traits_8hpp.html">boost/hana/traits.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="type_8hpp.html">boost/hana/type.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;type_traits&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;utility&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> ...&gt;</div><div class="line"><span class="keyword">using</span> void_t = void;</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T, <span class="keyword">typename</span> = <span class="keywordtype">void</span>&gt;</div><div class="line"><span class="keyword">struct </span>has_type : std::false_type { };</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T&gt;</div><div class="line"><span class="keyword">struct </span>has_type&lt;T, void_t&lt;typename T::type&gt;&gt;</div><div class="line">    : std::true_type</div><div class="line">{ };</div><div class="line"></div><div class="line"><span class="keyword">auto</span> common_type_impl = hana::sfinae([](<span class="keyword">auto</span> t, <span class="keyword">auto</span> u) -&gt; hana::type&lt;</div><div class="line">    decltype(<span class="keyword">true</span> ? hana::traits::declval(t) : hana::traits::declval(u))</div><div class="line">&gt; { <span class="keywordflow">return</span> {}; });</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T, <span class="keyword">typename</span> U&gt;</div><div class="line"><span class="keyword">using</span> common_type = decltype(common_type_impl(hana::type_c&lt;T&gt;, hana::type_c&lt;U&gt;));</div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(</div><div class="line">  common_type_impl(hana::type_c&lt;int&gt;, hana::type_c&lt;float&gt;)</div><div class="line">    ==</div><div class="line">  hana::just(hana::type_c&lt;float&gt;)</div><div class="line">);</div><div class="line"></div><div class="line">static_assert(!has_type&lt;common_type&lt;int, int*&gt;&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(std::is_same&lt;common_type&lt;int, float&gt;::type, <span class="keywordtype">float</span>&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --><h2>Modeled concepts </h2>
    149. 

  149. <ol type="1">
    150. 

  150. <li><code>Comparable</code><br />
    151. 

  151. Two <code>optional</code>s are equal if and only if they are both empty or they both contain a value and those values are equal. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="not__equal_8hpp.html">boost/hana/not_equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::nothing == hana::nothing);</div><div class="line">static_assert(hana::just(<span class="charliteral">&#39;x&#39;</span>) == hana::just(<span class="charliteral">&#39;x&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(hana::just(<span class="charliteral">&#39;x&#39;</span>) != hana::just(<span class="charliteral">&#39;y&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::just(<span class="charliteral">&#39;x&#39;</span>) != hana::nothing);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --></li>
    152. 

  152. <li><code>Orderable</code><br />
    153. 

  153. Optional values can be ordered by considering the value they are holding, if any. To handle the case of an empty optional value, we arbitrarily set <code>nothing</code> as being less than any other <code>just</code>. Hence, <div class="fragment"><div class="line"><a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x) &lt; <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(y) <span class="keywordflow">if</span> and only <span class="keywordflow">if</span> x &lt; y</div><div class="line"><a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a> &lt; <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(anything)</div></div><!-- fragment --> Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="greater_8hpp.html">boost/hana/greater.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="less_8hpp.html">boost/hana/less.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::nothing &lt; hana::just(3));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::just(0) &gt; hana::nothing);</div><div class="line">static_assert(hana::just(1) &lt; hana::just(3), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(hana::just(3) &gt; hana::just(2), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --></li>
    154. 

  154. <li><code>Functor</code><br />
    155. 

  155. An optional value can be seen as a list containing either one element (<code>just(x)</code>) or no elements at all (<code>nothing</code>). As such, mapping a function over an optional value is equivalent to applying it to its value if there is one, and to <code>nothing</code> otherwise: <div class="fragment"><div class="line"><a class="code" href="group__group-Functor.html#ga5a8975f6e55375d5b6038a9c36ee63e7">transform</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x), f) == <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(f(x))</div><div class="line"><a class="code" href="group__group-Functor.html#ga5a8975f6e55375d5b6038a9c36ee63e7">transform</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>, f) == <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a></div></div><!-- fragment --> Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="placeholder_8hpp.html">boost/hana/functional/placeholder.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="transform_8hpp.html">boost/hana/transform.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Functor.html#ga5a8975f6e55375d5b6038a9c36ee63e7">hana::transform</a>(hana::nothing, <a class="code" href="group__group-functional.html#gaefe9fd152cba94be71c2b5b9de689d23">hana::_</a> + 1) == hana::nothing);</div><div class="line">static_assert(<a class="code" href="group__group-Functor.html#ga5a8975f6e55375d5b6038a9c36ee63e7">hana::transform</a>(hana::just(1), <a class="code" href="group__group-functional.html#gaefe9fd152cba94be71c2b5b9de689d23">hana::_</a> + 1) == hana::just(2), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --></li>
    156. 

  156. <li><code>Applicative</code><br />
    157. 

  157. First, a value can be made optional with <code>lift&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html" title="Tag representing a hana::optional.">optional_tag</a>&gt;</code>, which is equivalent to <code>just</code>. Second, one can feed an optional value to an optional function with <code>ap</code>, which will return <code>just(f(x))</code> if there is both a function <em>and</em> a value, and <code>nothing</code> otherwise: <div class="fragment"><div class="line"><a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">ap</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(f), <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x)) == <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(f(x))</div><div class="line"><a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">ap</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>, <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x)) == <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a></div><div class="line"><a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">ap</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(f), <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>) == <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a></div><div class="line"><a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">ap</a>(<a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>, <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>) == <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a></div></div><!-- fragment --> A simple example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="ap_8hpp.html">boost/hana/ap.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">constexpr <span class="keywordtype">char</span> next(<span class="keywordtype">char</span> c) { <span class="keywordflow">return</span> c + 1; }</div><div class="line"></div><div class="line">static_assert(<a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">hana::ap</a>(hana::just(next), hana::just(<span class="charliteral">&#39;x&#39;</span>)) == hana::just(<span class="charliteral">&#39;y&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">hana::ap</a>(hana::nothing, hana::just(<span class="charliteral">&#39;x&#39;</span>)) == hana::nothing);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">hana::ap</a>(hana::just(next), hana::nothing) == hana::nothing);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">hana::ap</a>(hana::nothing, hana::nothing) == hana::nothing);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> A more complex example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="ap_8hpp.html">boost/hana/ap.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="bool_8hpp.html">boost/hana/bool.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="config_8hpp.html">boost/hana/config.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="if_8hpp.html">boost/hana/if.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="lift_8hpp.html">boost/hana/lift.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keywordtype">char</span> op&gt;</div><div class="line">constexpr <span class="keyword">auto</span> <span class="keyword">function</span> = hana::nothing;</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;&gt;</div><div class="line">BOOST_HANA_CONSTEXPR_LAMBDA <span class="keyword">auto</span> function&lt;&#39;+&#39;&gt; = hana::just([](<span class="keyword">auto</span> x, <span class="keyword">auto</span> y) {</div><div class="line">    <span class="keywordflow">return</span> x + y;</div><div class="line">});</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;&gt;</div><div class="line">BOOST_HANA_CONSTEXPR_LAMBDA <span class="keyword">auto</span> <span class="keyword">function</span>&lt;<span class="charliteral">&#39;-&#39;</span>&gt; = hana::just([](<span class="keyword">auto</span> x, <span class="keyword">auto</span> y) {</div><div class="line">    <span class="keywordflow">return</span> x - y;</div><div class="line">});</div><div class="line"></div><div class="line"><span class="comment">// and so on...</span></div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keywordtype">char</span> n&gt;</div><div class="line">constexpr <span class="keyword">auto</span> digit = <a class="code" href="group__group-Logical.html#gafd655d2222367131e7a63616e93dd080">hana::if_</a>(hana::bool_c&lt;(n &gt;= <span class="charliteral">&#39;0&#39;</span> &amp;&amp; n &lt;= <span class="charliteral">&#39;9&#39;</span>)&gt;,</div><div class="line">    hana::just(static_cast&lt;int&gt;(n - 48)),</div><div class="line">    hana::nothing</div><div class="line">);</div><div class="line"></div><div class="line"><span class="keyword">template</span> &lt;<span class="keywordtype">char</span> x, <span class="keywordtype">char</span> op, <span class="keywordtype">char</span> y&gt;</div><div class="line">BOOST_HANA_CONSTEXPR_LAMBDA <span class="keyword">auto</span> evaluate = <a class="code" href="group__group-Applicative.html#ga6176a32953dbbdee96e3ab3c73e4c890">hana::ap</a>(function&lt;op&gt;, digit&lt;x&gt;, digit&lt;y&gt;);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    <a class="code" href="group__group-assertions.html#gac7aafc41e4dcc7d1f1929fb00f010d2a">BOOST_HANA_CONSTEXPR_CHECK</a>(evaluate&lt;&#39;1&#39;, &#39;+&#39;, &#39;2&#39;&gt; == hana::just(1 + 2));</div><div class="line">    <a class="code" href="group__group-assertions.html#gac7aafc41e4dcc7d1f1929fb00f010d2a">BOOST_HANA_CONSTEXPR_CHECK</a>(evaluate&lt;&#39;4&#39;, &#39;-&#39;, &#39;2&#39;&gt; == hana::just(4 - 2));</div><div class="line"></div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(evaluate&lt;&#39;?&#39;, &#39;+&#39;, &#39;2&#39;&gt; == hana::nothing);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(evaluate&lt;&#39;1&#39;, &#39;?&#39;, &#39;2&#39;&gt; == hana::nothing);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(evaluate&lt;&#39;1&#39;, &#39;+&#39;, &#39;?&#39;&gt; == hana::nothing);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(evaluate&lt;&#39;?&#39;, &#39;?&#39;, &#39;?&#39;&gt; == hana::nothing);</div><div class="line"></div><div class="line">    static_assert(hana::lift&lt;hana::optional_tag&gt;(123) == hana::just(123), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">}</div></div><!-- fragment --></li>
    158. 

  158. <li><code>Monad</code><br />
    159. 

  159. The <code>Monad</code> model makes it easy to compose actions that might fail. One can feed an optional value if there is one into a function with <code>chain</code>, which will return <code>nothing</code> if there is no value. Finally, optional-optional values can have their redundant level of optionality removed with <code>flatten</code>. Also note that the <code>|</code> operator can be used in place of the <code>chain</code> function. Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="chain_8hpp.html">boost/hana/chain.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="config_8hpp.html">boost/hana/config.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="flatten_8hpp.html">boost/hana/flatten.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    BOOST_HANA_CONSTEXPR_LAMBDA <span class="keyword">auto</span> inc = [](<span class="keyword">auto</span> x) {</div><div class="line">        <span class="keywordflow">return</span> hana::just(x + 1);</div><div class="line">    };</div><div class="line"></div><div class="line">    <a class="code" href="group__group-assertions.html#gac7aafc41e4dcc7d1f1929fb00f010d2a">BOOST_HANA_CONSTEXPR_CHECK</a>(<a class="code" href="group__group-Monad.html#ga05fdc2f887cb9967357487a432ace98b">hana::chain</a>(hana::just(1), inc) == hana::just(2));</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Monad.html#ga05fdc2f887cb9967357487a432ace98b">hana::chain</a>(hana::nothing, inc) == hana::nothing);</div><div class="line"></div><div class="line">    <a class="code" href="group__group-assertions.html#gac7aafc41e4dcc7d1f1929fb00f010d2a">BOOST_HANA_CONSTEXPR_CHECK</a>(<a class="code" href="group__group-Monad.html#gaa5fec6fb418de5da3ecb500fd6cc54dd">hana::flatten</a>(hana::just(hana::just(2))) == hana::just(2));</div><div class="line">}</div></div><!-- fragment --></li>
    160. 

  160. <li><code>MonadPlus</code><br />
    161. 

  161. The <code>MonadPlus</code> model allows choosing the first valid value out of two optional values with <code>concat</code>. If both optional values are <code>nothing</code>s, <code>concat</code> will return <code>nothing</code>. Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="concat_8hpp.html">boost/hana/concat.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="empty_8hpp.html">boost/hana/empty.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">static_assert(<a class="code" href="group__group-MonadPlus.html#ga1946e96c3b4c178c7ae8703724c29c37">hana::concat</a>(hana::nothing, hana::just(<span class="charliteral">&#39;x&#39;</span>)) == hana::just(<span class="charliteral">&#39;x&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-MonadPlus.html#ga1946e96c3b4c178c7ae8703724c29c37">hana::concat</a>(hana::nothing, hana::nothing) == hana::nothing);</div><div class="line">static_assert(<a class="code" href="group__group-MonadPlus.html#ga1946e96c3b4c178c7ae8703724c29c37">hana::concat</a>(hana::just(<span class="charliteral">&#39;x&#39;</span>), hana::just(<span class="charliteral">&#39;y&#39;</span>)) == hana::just(<span class="charliteral">&#39;x&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::empty&lt;hana::optional_tag&gt;() == hana::nothing);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --></li>
    162. 

  162. <li><code>Foldable</code><br />
    163. 

  163. Folding an optional value is equivalent to folding a list containing either no elements (for <code>nothing</code>) or <code>x</code> (for <code>just(x)</code>). Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="fold__right_8hpp.html">boost/hana/fold_right.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="plus_8hpp.html">boost/hana/plus.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">static_assert(<a class="code" href="group__group-Foldable.html#ga77d43badebd59b046cf93598733649b4">hana::fold_right</a>(hana::nothing, 1, <a class="code" href="group__group-Monoid.html#gaeb5d4a1e967e319712f9e4791948896c">hana::plus</a>) == 1, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(<a class="code" href="group__group-Foldable.html#ga77d43badebd59b046cf93598733649b4">hana::fold_right</a>(hana::just(4), 1, <a class="code" href="group__group-Monoid.html#gaeb5d4a1e967e319712f9e4791948896c">hana::plus</a>) == 5, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --></li>
    164. 

  164. <li><code>Searchable</code><br />
    165. 

  165. Searching an optional value is equivalent to searching a list containing <code>x</code> for <code>just(x)</code> and an empty list for <code>nothing</code>. Example: <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="all__of_8hpp.html">boost/hana/all_of.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="find__if_8hpp.html">boost/hana/find_if.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="integral__constant_8hpp.html">boost/hana/integral_constant.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="mod_8hpp.html">boost/hana/mod.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="not__equal_8hpp.html">boost/hana/not_equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keyword">auto</span> odd = [](<span class="keyword">auto</span> x) {</div><div class="line">    <span class="keywordflow">return</span> x % hana::int_c&lt;2&gt; != hana::int_c&lt;0&gt;;</div><div class="line">};</div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Searchable.html#ga7f99b80672aa80a7eb8b223955ce546f">hana::find_if</a>(hana::just(hana::int_c&lt;3&gt;), odd) == hana::just(hana::int_c&lt;3&gt;));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Searchable.html#ga7f99b80672aa80a7eb8b223955ce546f">hana::find_if</a>(hana::just(hana::int_c&lt;2&gt;), odd) == hana::nothing);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Searchable.html#ga7f99b80672aa80a7eb8b223955ce546f">hana::find_if</a>(hana::nothing, odd) == hana::nothing);</div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Searchable.html#ga3a168950082f38afd9edf256f336c8ba">hana::all_of</a>(hana::just(hana::int_c&lt;3&gt;), odd));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(<a class="code" href="group__group-Searchable.html#ga3a168950082f38afd9edf256f336c8ba">hana::all_of</a>(hana::nothing, odd));</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> </li>
    166. 

  166. </ol>
    167. 

  167. </div><table class="memberdecls">
    168. 

  168. <tr class="heading"><td colspan="2"><h2 class="groupheader"><a name="related"></a>
    169. 

  169. Synopsis of associated functions</h2></td></tr>
    170. 

  170. <tr class="memitem:a76a84854b3051af000e698157d63a6a8"><td class="memTemplParams" colspan="2">template&lt;&gt; </td></tr>
    171. 

  171. <tr class="memitem:a76a84854b3051af000e698157d63a6a8"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a76a84854b3051af000e698157d63a6a8">make&lt; optional_tag &gt;</a></td></tr>
    172. 

  172. <tr class="memdesc:a76a84854b3051af000e698157d63a6a8"><td class="mdescLeft">&#160;</td><td class="mdescRight">Create an optional value.  <a href="#a76a84854b3051af000e698157d63a6a8">More...</a><br /></td></tr>
    173. 

  173. <tr class="separator:a76a84854b3051af000e698157d63a6a8"><td class="memSeparator" colspan="2">&#160;</td></tr>
    174. 

  174. <tr class="memitem:af1105a435d6435a2437cb5dc162ba463"><td class="memItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#af1105a435d6435a2437cb5dc162ba463">make_optional</a> = <a class="el" href="group__group-core.html#ga1d92480f0af1029878e773dafa3e2f60">make</a>&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html">optional_tag</a>&gt;</td></tr>
    175. 

  175. <tr class="memdesc:af1105a435d6435a2437cb5dc162ba463"><td class="mdescLeft">&#160;</td><td class="mdescRight">Alias to <code>make&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html" title="Tag representing a hana::optional.">optional_tag</a>&gt;</code>; provided for convenience.  <a href="#af1105a435d6435a2437cb5dc162ba463">More...</a><br /></td></tr>
    176. 

  176. <tr class="separator:af1105a435d6435a2437cb5dc162ba463"><td class="memSeparator" colspan="2">&#160;</td></tr>
    177. 

  177. <tr class="memitem:a8ad818d6df7379b0adefa45d2ac844d0"><td class="memItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a></td></tr>
    178. 

  178. <tr class="memdesc:a8ad818d6df7379b0adefa45d2ac844d0"><td class="mdescLeft">&#160;</td><td class="mdescRight">Create an optional value containing <code>x</code>.  <a href="#a8ad818d6df7379b0adefa45d2ac844d0">More...</a><br /></td></tr>
    179. 

  179. <tr class="separator:a8ad818d6df7379b0adefa45d2ac844d0"><td class="memSeparator" colspan="2">&#160;</td></tr>
    180. 

  180. <tr class="memitem:a0c315287473aa87fe5ccd9d8a1c10186"><td class="memItemLeft" align="right" valign="top">constexpr <a class="el" href="structboost_1_1hana_1_1optional.html">optional</a>&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a> {}</td></tr>
    181. 

  181. <tr class="memdesc:a0c315287473aa87fe5ccd9d8a1c10186"><td class="mdescLeft">&#160;</td><td class="mdescRight">An empty optional value.  <a href="#a0c315287473aa87fe5ccd9d8a1c10186">More...</a><br /></td></tr>
    182. 

  182. <tr class="separator:a0c315287473aa87fe5ccd9d8a1c10186"><td class="memSeparator" colspan="2">&#160;</td></tr>
    183. 

  183. <tr class="memitem:a6529e11cb228e99a05e81071752807d5"><td class="memItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a6529e11cb228e99a05e81071752807d5">maybe</a></td></tr>
    184. 

  184. <tr class="memdesc:a6529e11cb228e99a05e81071752807d5"><td class="mdescLeft">&#160;</td><td class="mdescRight">Apply a function to the contents of an optional, with a fallback result.  <a href="#a6529e11cb228e99a05e81071752807d5">More...</a><br /></td></tr>
    185. 

  185. <tr class="separator:a6529e11cb228e99a05e81071752807d5"><td class="memSeparator" colspan="2">&#160;</td></tr>
    186. 

  186. <tr class="memitem:aa4c8c0b1ef3d8206974734b93646c9fd"><td class="memItemLeft" align="right" valign="top">auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#aa4c8c0b1ef3d8206974734b93646c9fd">sfinae</a></td></tr>
    187. 

  187. <tr class="memdesc:aa4c8c0b1ef3d8206974734b93646c9fd"><td class="mdescLeft">&#160;</td><td class="mdescRight">Calls a function if the call expression is well-formed.  <a href="#aa4c8c0b1ef3d8206974734b93646c9fd">More...</a><br /></td></tr>
    188. 

  188. <tr class="separator:aa4c8c0b1ef3d8206974734b93646c9fd"><td class="memSeparator" colspan="2">&#160;</td></tr>
    189. 

  189. <tr class="memitem:a1047a69893a62cd36eb0d2934f85e0e7"><td class="memItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a1047a69893a62cd36eb0d2934f85e0e7">is_just</a></td></tr>
    190. 

  190. <tr class="memdesc:a1047a69893a62cd36eb0d2934f85e0e7"><td class="mdescLeft">&#160;</td><td class="mdescRight">Return whether an <code>optional</code> contains a value.  <a href="#a1047a69893a62cd36eb0d2934f85e0e7">More...</a><br /></td></tr>
    191. 

  191. <tr class="separator:a1047a69893a62cd36eb0d2934f85e0e7"><td class="memSeparator" colspan="2">&#160;</td></tr>
    192. 

  192. <tr class="memitem:a4eb266d123883c23853f7731a6c5953f"><td class="memItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a4eb266d123883c23853f7731a6c5953f">is_nothing</a></td></tr>
    193. 

  193. <tr class="memdesc:a4eb266d123883c23853f7731a6c5953f"><td class="mdescLeft">&#160;</td><td class="mdescRight">Return whether an <code>optional</code> is empty.  <a href="#a4eb266d123883c23853f7731a6c5953f">More...</a><br /></td></tr>
    194. 

  194. <tr class="separator:a4eb266d123883c23853f7731a6c5953f"><td class="memSeparator" colspan="2">&#160;</td></tr>
    195. 

  195. </table><table class="memberdecls">
    196. 

  196. <tr class="heading"><td colspan="2"><h2 class="groupheader"><a name="friends"></a>
    197. 

  197. Friends</h2></td></tr>
    198. 

  198. <tr class="memitem:a833b9d5d44bb431621af9129994b9bbc"><td class="memTemplParams" colspan="2"><a id="a833b9d5d44bb431621af9129994b9bbc"></a>
    199. 

  199. template&lt;typename ... T, typename F &gt; </td></tr>
    200. 

  200. <tr class="memitem:a833b9d5d44bb431621af9129994b9bbc"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a833b9d5d44bb431621af9129994b9bbc">operator|</a> (<a class="el" href="structboost_1_1hana_1_1optional.html">optional</a>&lt; T... &gt;, F)</td></tr>
    201. 

  201. <tr class="memdesc:a833b9d5d44bb431621af9129994b9bbc"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Monad.html#ga05fdc2f887cb9967357487a432ace98b" title="Feed a monadic value into a monadic computation.Given a monadic value and a monadic function,...">hana::chain</a></code>. <br /></td></tr>
    202. 

  202. <tr class="separator:a833b9d5d44bb431621af9129994b9bbc"><td class="memSeparator" colspan="2">&#160;</td></tr>
    203. 

  203. <tr class="memitem:a27607958295e6da8a0ba602a2db468f1"><td class="memTemplParams" colspan="2"><a id="a27607958295e6da8a0ba602a2db468f1"></a>
    204. 

  204. template&lt;typename X , typename Y &gt; </td></tr>
    205. 

  205. <tr class="memitem:a27607958295e6da8a0ba602a2db468f1"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a27607958295e6da8a0ba602a2db468f1">operator==</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    206. 

  206. <tr class="memdesc:a27607958295e6da8a0ba602a2db468f1"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Comparable.html#gacaf1ebea6b3ab96ac9dcb82f0e64e547" title="Returns a Logical representing whether x is equal to y.The equal function can be called in two differ...">hana::equal</a></code> <br /></td></tr>
    207. 

  207. <tr class="separator:a27607958295e6da8a0ba602a2db468f1"><td class="memSeparator" colspan="2">&#160;</td></tr>
    208. 

  208. <tr class="memitem:a0cae3af55edb3eb05bfa184bda633f7d"><td class="memTemplParams" colspan="2"><a id="a0cae3af55edb3eb05bfa184bda633f7d"></a>
    209. 

  209. template&lt;typename X , typename Y &gt; </td></tr>
    210. 

  210. <tr class="memitem:a0cae3af55edb3eb05bfa184bda633f7d"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a0cae3af55edb3eb05bfa184bda633f7d">operator!=</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    211. 

  211. <tr class="memdesc:a0cae3af55edb3eb05bfa184bda633f7d"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Comparable.html#gae33be2e0d5e04f19082f4b7740dfc9cd" title="Returns a Logical representing whether x is not equal to y.The not_equal function can be called in tw...">hana::not_equal</a></code> <br /></td></tr>
    212. 

  212. <tr class="separator:a0cae3af55edb3eb05bfa184bda633f7d"><td class="memSeparator" colspan="2">&#160;</td></tr>
    213. 

  213. <tr class="memitem:ada078dcf9e68b8e9a82b33c75dde8457"><td class="memTemplParams" colspan="2"><a id="ada078dcf9e68b8e9a82b33c75dde8457"></a>
    214. 

  214. template&lt;typename X , typename Y &gt; </td></tr>
    215. 

  215. <tr class="memitem:ada078dcf9e68b8e9a82b33c75dde8457"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#ada078dcf9e68b8e9a82b33c75dde8457">operator&lt;</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    216. 

  216. <tr class="memdesc:ada078dcf9e68b8e9a82b33c75dde8457"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Orderable.html#gad510011602bdb14686f1c4ec145301c9" title="Returns a Logical representing whether x is less than y.">hana::less</a></code> <br /></td></tr>
    217. 

  217. <tr class="separator:ada078dcf9e68b8e9a82b33c75dde8457"><td class="memSeparator" colspan="2">&#160;</td></tr>
    218. 

  218. <tr class="memitem:aedd11fddefddbbbb87d3b24c7117528f"><td class="memTemplParams" colspan="2"><a id="aedd11fddefddbbbb87d3b24c7117528f"></a>
    219. 

  219. template&lt;typename X , typename Y &gt; </td></tr>
    220. 

  220. <tr class="memitem:aedd11fddefddbbbb87d3b24c7117528f"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#aedd11fddefddbbbb87d3b24c7117528f">operator&gt;</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    221. 

  221. <tr class="memdesc:aedd11fddefddbbbb87d3b24c7117528f"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Orderable.html#gaf9a073eafebbe514fb19dff82318f198" title="Returns a Logical representing whether x is greater than y.">hana::greater</a></code> <br /></td></tr>
    222. 

  222. <tr class="separator:aedd11fddefddbbbb87d3b24c7117528f"><td class="memSeparator" colspan="2">&#160;</td></tr>
    223. 

  223. <tr class="memitem:a53817979990da799e659b2576799a79a"><td class="memTemplParams" colspan="2"><a id="a53817979990da799e659b2576799a79a"></a>
    224. 

  224. template&lt;typename X , typename Y &gt; </td></tr>
    225. 

  225. <tr class="memitem:a53817979990da799e659b2576799a79a"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a53817979990da799e659b2576799a79a">operator&lt;=</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    226. 

  226. <tr class="memdesc:a53817979990da799e659b2576799a79a"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Orderable.html#ga9917dd82beb67151bf5657245d37b851" title="Returns a Logical representing whether x is less than or equal to y.">hana::less_equal</a></code> <br /></td></tr>
    227. 

  227. <tr class="separator:a53817979990da799e659b2576799a79a"><td class="memSeparator" colspan="2">&#160;</td></tr>
    228. 

  228. <tr class="memitem:aa3f0436b623de5208767751e1e9f7e59"><td class="memTemplParams" colspan="2"><a id="aa3f0436b623de5208767751e1e9f7e59"></a>
    229. 

  229. template&lt;typename X , typename Y &gt; </td></tr>
    230. 

  230. <tr class="memitem:aa3f0436b623de5208767751e1e9f7e59"><td class="memTemplItemLeft" align="right" valign="top">constexpr auto&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#aa3f0436b623de5208767751e1e9f7e59">operator&gt;=</a> (X &amp;&amp;x, Y &amp;&amp;y)</td></tr>
    231. 

  231. <tr class="memdesc:aa3f0436b623de5208767751e1e9f7e59"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="group__group-Orderable.html#ga6023631e7d0a01e16dc3fa4221fbd703" title="Returns a Logical representing whether x is greater than or equal to y.">hana::greater_equal</a></code> <br /></td></tr>
    232. 

  232. <tr class="separator:aa3f0436b623de5208767751e1e9f7e59"><td class="memSeparator" colspan="2">&#160;</td></tr>
    233. 

  233. </table><table class="memberdecls">
    234. 

  234. <tr class="heading"><td colspan="2"><h2 class="groupheader"><a name="pub-methods"></a>
    235. 

  235. Public Member Functions</h2></td></tr>
    236. 

  236. <tr class="memitem:aefbd278cf7bb9e921707d6d827014bcb"><td class="memItemLeft" align="right" valign="top"><a id="aefbd278cf7bb9e921707d6d827014bcb"></a>
    237. 

  237. constexpr&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#aefbd278cf7bb9e921707d6d827014bcb">optional</a> ()=default</td></tr>
    238. 

  238. <tr class="memdesc:aefbd278cf7bb9e921707d6d827014bcb"><td class="mdescLeft">&#160;</td><td class="mdescRight">Default-construct an <code>optional</code>. Only exists if the optional contains a value, and if that value is DefaultConstructible. <br /></td></tr>
    239. 

  239. <tr class="separator:aefbd278cf7bb9e921707d6d827014bcb"><td class="memSeparator" colspan="2">&#160;</td></tr>
    240. 

  240. <tr class="memitem:a9074406adb54a6a848ba482895c51d1f"><td class="memItemLeft" align="right" valign="top"><a id="a9074406adb54a6a848ba482895c51d1f"></a>
    241. 

  241. &#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a9074406adb54a6a848ba482895c51d1f">optional</a> (<a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> const &amp;)=default</td></tr>
    242. 

  242. <tr class="memdesc:a9074406adb54a6a848ba482895c51d1f"><td class="mdescLeft">&#160;</td><td class="mdescRight">Copy-construct an <code>optional</code>. An empty optional may only be copy-constructed from another empty <code>optional</code>, and an <code>optional</code> with a value may only be copy-constructed from another <code>optional</code> with a value. Furthermore, this constructor only exists if the value held in the <code>optional</code> is CopyConstructible. <br /></td></tr>
    243. 

  243. <tr class="separator:a9074406adb54a6a848ba482895c51d1f"><td class="memSeparator" colspan="2">&#160;</td></tr>
    244. 

  244. <tr class="memitem:ac46a23f0ada20175c4c9a58af3d15753"><td class="memItemLeft" align="right" valign="top"><a id="ac46a23f0ada20175c4c9a58af3d15753"></a>
    245. 

  245. &#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#ac46a23f0ada20175c4c9a58af3d15753">optional</a> (<a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> &amp;&amp;)=default</td></tr>
    246. 

  246. <tr class="memdesc:ac46a23f0ada20175c4c9a58af3d15753"><td class="mdescLeft">&#160;</td><td class="mdescRight">Move-construct an <code>optional</code>. An empty optional may only be move-constructed from another empty <code>optional</code>, and an <code>optional</code> with a value may only be move-constructed from another <code>optional</code> with a value. Furthermore, this constructor only exists if the value held in the <code>optional</code> is MoveConstructible. <br /></td></tr>
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  248. <tr class="memitem:a2b04ee87c0ac58621edf3ad5476a2949"><td class="memItemLeft" align="right" valign="top"><a id="a2b04ee87c0ac58621edf3ad5476a2949"></a>
    249. 

  249. constexpr&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a2b04ee87c0ac58621edf3ad5476a2949">optional</a> (T const &amp;t)</td></tr>
    250. 

  250. <tr class="memdesc:a2b04ee87c0ac58621edf3ad5476a2949"><td class="mdescLeft">&#160;</td><td class="mdescRight">Construct an <code>optional</code> holding a value of type <code>T</code> from another object of type <code>T</code>. The value is copy-constructed. <br /></td></tr>
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    253. 

  253. constexpr&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#adf824827b5d6f8c87d23b9526b9f35d2">optional</a> (T &amp;&amp;t)</td></tr>
    254. 

  254. <tr class="memdesc:adf824827b5d6f8c87d23b9526b9f35d2"><td class="mdescLeft">&#160;</td><td class="mdescRight">Construct an <code>optional</code> holding a value of type <code>T</code> from another object of type <code>T</code>. The value is move-constructed. <br /></td></tr>
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  256. <tr class="memitem:abefc885243f38c66c52fdada3588f951"><td class="memItemLeft" align="right" valign="top"><a id="abefc885243f38c66c52fdada3588f951"></a>
    257. 

  257. constexpr <a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> &amp;&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#abefc885243f38c66c52fdada3588f951">operator=</a> (<a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> const &amp;)=default</td></tr>
    258. 

  258. <tr class="memdesc:abefc885243f38c66c52fdada3588f951"><td class="mdescLeft">&#160;</td><td class="mdescRight">Copy-assign an <code>optional</code>. An empty optional may only be copy-assigned from another empty <code>optional</code>, and an <code>optional</code> with a value may only be copy-assigned from another <code>optional</code> with a value. Furthermore, this assignment operator only exists if the value held in the <code>optional</code> is CopyAssignable. <br /></td></tr>
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  260. <tr class="memitem:a52057c5da4b9aee49ee15a167459fc9a"><td class="memItemLeft" align="right" valign="top"><a id="a52057c5da4b9aee49ee15a167459fc9a"></a>
    261. 

  261. constexpr <a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> &amp;&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a52057c5da4b9aee49ee15a167459fc9a">operator=</a> (<a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> &amp;&amp;)=default</td></tr>
    262. 

  262. <tr class="memdesc:a52057c5da4b9aee49ee15a167459fc9a"><td class="mdescLeft">&#160;</td><td class="mdescRight">Move-assign an <code>optional</code>. An empty optional may only be move-assigned from another empty <code>optional</code>, and an <code>optional</code> with a value may only be move-assigned from another <code>optional</code> with a value. Furthermore, this assignment operator only exists if the value held in the <code>optional</code> is MoveAssignable. <br /></td></tr>
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  264. <tr class="memitem:a6eeef1512294d408857805eb3a790230"><td class="memItemLeft" align="right" valign="top">constexpr T *&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a6eeef1512294d408857805eb3a790230">operator-&gt;</a> ()</td></tr>
    265. 

  265. <tr class="memdesc:a6eeef1512294d408857805eb3a790230"><td class="mdescLeft">&#160;</td><td class="mdescRight">Returns a pointer to the contained value, or a <code>nullptr</code> if the <code>optional</code> is empty.  <a href="#a6eeef1512294d408857805eb3a790230">More...</a><br /></td></tr>
    266. 

  266. <tr class="separator:a6eeef1512294d408857805eb3a790230"><td class="memSeparator" colspan="2">&#160;</td></tr>
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  267. <tr class="memitem:acdcf71b43d4e27d5efbf0ad875d0be58"><td class="memItemLeft" align="right" valign="top">constexpr T &amp;&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58">value</a> ()</td></tr>
    268. 

  268. <tr class="memdesc:acdcf71b43d4e27d5efbf0ad875d0be58"><td class="mdescLeft">&#160;</td><td class="mdescRight">Extract the content of an <code>optional</code>, or fail at compile-time.  <a href="#acdcf71b43d4e27d5efbf0ad875d0be58">More...</a><br /></td></tr>
    269. 

  269. <tr class="separator:acdcf71b43d4e27d5efbf0ad875d0be58"><td class="memSeparator" colspan="2">&#160;</td></tr>
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  270. <tr class="memitem:a237b2147dd9b72bb2dfa4a268056e4c2"><td class="memItemLeft" align="right" valign="top">constexpr T &amp;&#160;</td><td class="memItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a237b2147dd9b72bb2dfa4a268056e4c2">operator *</a> ()</td></tr>
    271. 

  271. <tr class="memdesc:a237b2147dd9b72bb2dfa4a268056e4c2"><td class="mdescLeft">&#160;</td><td class="mdescRight">Equivalent to <code><a class="el" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58" title="Extract the content of an optional, or fail at compile-time.">value()</a></code>, provided for convenience.  <a href="#a237b2147dd9b72bb2dfa4a268056e4c2">More...</a><br /></td></tr>
    272. 

  272. <tr class="separator:a237b2147dd9b72bb2dfa4a268056e4c2"><td class="memSeparator" colspan="2">&#160;</td></tr>
    273. 

  273. <tr class="memitem:a1f511267d94c178e4ef955da60f0888d"><td class="memTemplParams" colspan="2">template&lt;typename U &gt; </td></tr>
    274. 

  274. <tr class="memitem:a1f511267d94c178e4ef955da60f0888d"><td class="memTemplItemLeft" align="right" valign="top">decltype(auto) constexpr&#160;</td><td class="memTemplItemRight" valign="bottom"><a class="el" href="structboost_1_1hana_1_1optional.html#a1f511267d94c178e4ef955da60f0888d">value_or</a> (U &amp;&amp;<a class="el" href="structboost_1_1hana_1_1default__.html">default_</a>)</td></tr>
    275. 

  275. <tr class="memdesc:a1f511267d94c178e4ef955da60f0888d"><td class="mdescLeft">&#160;</td><td class="mdescRight">Return the contents of an <code>optional</code>, with a fallback result.  <a href="#a1f511267d94c178e4ef955da60f0888d">More...</a><br /></td></tr>
    276. 

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    277. 

  277. </table>
    278. 

  278. <h2 class="groupheader">Associated functions</h2>
    279. 

  279. <a id="a76a84854b3051af000e698157d63a6a8"></a>
    280. 

  280. <h2 class="memtitle"><span class="permalink"><a href="#a76a84854b3051af000e698157d63a6a8">&#9670;&nbsp;</a></span>make&lt; optional_tag &gt;</h2>
    281. 

  281. 

  282. <div class="memitem">
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  283. <div class="memproto">
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  284. <div class="memtemplate">
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  285. template&lt;typename ... T&gt; </div>
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  286. <div class="memtemplate">
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  287. template&lt;&gt; </div>
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  293.           <td class="memname">constexpr auto <a class="el" href="group__group-core.html#ga1d92480f0af1029878e773dafa3e2f60">make</a>&lt; <a class="el" href="structboost_1_1hana_1_1optional__tag.html">optional_tag</a> &gt;</td>
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  297.   <td class="mlabels-right">
    298. 

  298. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
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  299.   </tr>
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  300. </table>
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  301. </div><div class="memdoc">
    302. 

  302. <b>Initial value:</b><div class="fragment"><div class="line">= []([<span class="keyword">auto</span>&amp;&amp; x]) {</div><div class="line">        <span class="keywordflow">return</span> <a class="code" href="structboost_1_1hana_1_1optional.html#aefbd278cf7bb9e921707d6d827014bcb">optional</a>&lt;std::decay&lt;decltype(x)&gt;::type&gt;{forwarded(x)};</div><div class="line">    }</div></div><!-- fragment -->
    303. 

  303. <p>Create an optional value. </p>
    304. 

  304. <p>Specifically, <code>make&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html" title="Tag representing a hana::optional.">optional_tag</a>&gt;()</code> is equivalent to <code>nothing</code>, and <code>make&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html" title="Tag representing a hana::optional.">optional_tag</a>&gt;(x)</code> is equivalent to <code>just(x)</code>. This is provided for consistency with the other <code>make&lt;...&gt;</code> functions.</p>
    305. 

  305. <h2>Example </h2>
    306. 

  306. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2make_8hpp.html">boost/hana/core/make.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    constexpr <span class="keyword">auto</span> x = hana::make&lt;hana::optional_tag&gt;();</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(x == hana::make_optional());</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_nothing(x));</div><div class="line"></div><div class="line">    constexpr <span class="keyword">auto</span> just_x = hana::make&lt;hana::optional_tag&gt;(<span class="charliteral">&#39;x&#39;</span>);</div><div class="line">    static_assert(just_x == hana::make_optional(<span class="charliteral">&#39;x&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_just(just_x));</div><div class="line">}</div></div><!-- fragment --> 
    307. 

  307. </div>
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  308. </div>
    309. 

  309. <a id="af1105a435d6435a2437cb5dc162ba463"></a>
    310. 

  310. <h2 class="memtitle"><span class="permalink"><a href="#af1105a435d6435a2437cb5dc162ba463">&#9670;&nbsp;</a></span>make_optional</h2>
    311. 

  311. 

  312. <div class="memitem">
    313. 

  313. <div class="memproto">
    314. 

  314. <div class="memtemplate">
    315. 

  315. template&lt;typename ... T&gt; </div>
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  316. <table class="mlabels">
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  321.           <td class="memname">constexpr auto make_optional = <a class="el" href="group__group-core.html#ga1d92480f0af1029878e773dafa3e2f60">make</a>&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html">optional_tag</a>&gt;</td>
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  322.         </tr>
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    326. 

  326. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
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  327.   </tr>
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  328. </table>
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  329. </div><div class="memdoc">
    330. 

  330. 

  331. <p>Alias to <code>make&lt;<a class="el" href="structboost_1_1hana_1_1optional__tag.html" title="Tag representing a hana::optional.">optional_tag</a>&gt;</code>; provided for convenience. </p>
    332. 

  332. <h2>Example </h2>
    333. 

  333. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2make_8hpp.html">boost/hana/core/make.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    constexpr <span class="keyword">auto</span> x = hana::make&lt;hana::optional_tag&gt;();</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(x == hana::make_optional());</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_nothing(x));</div><div class="line"></div><div class="line">    constexpr <span class="keyword">auto</span> just_x = hana::make&lt;hana::optional_tag&gt;(<span class="charliteral">&#39;x&#39;</span>);</div><div class="line">    static_assert(just_x == hana::make_optional(<span class="charliteral">&#39;x&#39;</span>), <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_just(just_x));</div><div class="line">}</div></div><!-- fragment --> 
    334. 

  334. </div>
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  335. </div>
    336. 

  336. <a id="a8ad818d6df7379b0adefa45d2ac844d0"></a>
    337. 

  337. <h2 class="memtitle"><span class="permalink"><a href="#a8ad818d6df7379b0adefa45d2ac844d0">&#9670;&nbsp;</a></span>just</h2>
    338. 

  338. 

  339. <div class="memitem">
    340. 

  340. <div class="memproto">
    341. 

  341. <div class="memtemplate">
    342. 

  342. template&lt;typename ... T&gt; </div>
    343. 

  343. <table class="mlabels">
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    346. 

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    348. 

  348.           <td class="memname">constexpr auto just</td>
    349. 

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  351.   </td>
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  352.   <td class="mlabels-right">
    353. 

  353. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
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  354.   </tr>
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  355. </table>
    356. 

  356. </div><div class="memdoc">
    357. 

  357. <b>Initial value:</b><div class="fragment"><div class="line">= [](<span class="keyword">auto</span>&amp;&amp; x) {</div><div class="line">        <span class="keywordflow">return</span> <a class="code" href="structboost_1_1hana_1_1optional.html#aefbd278cf7bb9e921707d6d827014bcb">optional</a>&lt;std::decay&lt;decltype(x)&gt;::type&gt;{forwarded(x)};</div><div class="line">    }</div></div><!-- fragment -->
    358. 

  358. <p>Create an optional value containing <code>x</code>. </p>
    359. 

  359. <h2>Example </h2>
    360. 

  360. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">constexpr <span class="keyword">auto</span> just_x = hana::just(<span class="charliteral">&#39;x&#39;</span>);</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_just(just_x));</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    361. 

  361. </div>
    362. 

  362. </div>
    363. 

  363. <a id="a0c315287473aa87fe5ccd9d8a1c10186"></a>
    364. 

  364. <h2 class="memtitle"><span class="permalink"><a href="#a0c315287473aa87fe5ccd9d8a1c10186">&#9670;&nbsp;</a></span>nothing</h2>
    365. 

  365. 

  366. <div class="memitem">
    367. 

  367. <div class="memproto">
    368. 

  368. <div class="memtemplate">
    369. 

  369. template&lt;typename ... T&gt; </div>
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  375.           <td class="memname">constexpr <a class="el" href="structboost_1_1hana_1_1optional.html">optional</a> nothing {}</td>
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  382. </table>
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    384. 

  384. 

  385. <p>An empty optional value. </p>
    386. 

  386. <h2>Example </h2>
    387. 

  387. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">constexpr <span class="keyword">auto</span> x = hana::nothing;</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::is_nothing(x));</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    388. 

  388. </div>
    389. 

  389. </div>
    390. 

  390. <a id="a6529e11cb228e99a05e81071752807d5"></a>
    391. 

  391. <h2 class="memtitle"><span class="permalink"><a href="#a6529e11cb228e99a05e81071752807d5">&#9670;&nbsp;</a></span>maybe</h2>
    392. 

  392. 

  393. <div class="memitem">
    394. 

  394. <div class="memproto">
    395. 

  395. <div class="memtemplate">
    396. 

  396. template&lt;typename ... T&gt; </div>
    397. 

  397. <table class="mlabels">
    398. 

  398.   <tr>
    399. 

  399.   <td class="mlabels-left">
    400. 

  400.       <table class="memname">
    401. 

  401.         <tr>
    402. 

  402.           <td class="memname">constexpr auto maybe</td>
    403. 

  403.         </tr>
    404. 

  404.       </table>
    405. 

  405.   </td>
    406. 

  406.   <td class="mlabels-right">
    407. 

  407. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
    408. 

  408.   </tr>
    409. 

  409. </table>
    410. 

  410. </div><div class="memdoc">
    411. 

  411. <b>Initial value:</b><div class="fragment"><div class="line">= [](<span class="keyword">auto</span>&amp;&amp; default_, <span class="keyword">auto</span>&amp;&amp; f, <span class="keyword">auto</span>&amp;&amp; m) -&gt; decltype(<span class="keyword">auto</span>) {</div><div class="line">        <span class="keywordflow">if</span> (m is a <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x)) {</div><div class="line">            <span class="keywordflow">return</span> forwarded(f)(forwarded(x));</div><div class="line">        <span class="keywordflow">else</span></div><div class="line">            <span class="keywordflow">return</span> forwarded(default_);</div><div class="line">        }</div><div class="line">    }</div></div><!-- fragment -->
    412. 

  412. <p>Apply a function to the contents of an optional, with a fallback result. </p>
    413. 

  413. <p>Specifically, <code>maybe</code> takes a default value, a function and an optional value. If the optional value is <code>nothing</code>, the default value is returned. Otherwise, the function is applied to the content of the <code>just</code>.</p>
    414. 

  414. <dl class="params"><dt>Parameters</dt><dd>
    415. 

  415.   <table class="params">
    416. 

  416.     <tr><td class="paramname"><a class="el" href="structboost_1_1hana_1_1default__.html" title="Mark a tag-dispatched method implementation as a default implementation.">default_</a></td><td>A default value returned if <code>m</code> is <code>nothing</code>.</td></tr>
    417. 

  417.     <tr><td class="paramname">f</td><td>A function called as <code>f(x)</code> if and only if <code>m</code> is an optional value of the form <code>just(x)</code>. In that case, the result returend by <code>maybe</code> is the result of <code>f</code>.</td></tr>
    418. 

  418.     <tr><td class="paramname">m</td><td>An optional value.</td></tr>
    419. 

  419.   </table>
    420. 

  420.   </dd>
    421. 

  421. </dl>
    422. 

  422. <h2>Example </h2>
    423. 

  423. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="placeholder_8hpp.html">boost/hana/functional/placeholder.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">static_assert(hana::maybe(<span class="charliteral">&#39;x&#39;</span>, <a class="code" href="group__group-functional.html#gaefe9fd152cba94be71c2b5b9de689d23">hana::_</a> + 1, hana::just(1)) == 2, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(hana::maybe(<span class="charliteral">&#39;x&#39;</span>, <a class="code" href="group__group-functional.html#gaefe9fd152cba94be71c2b5b9de689d23">hana::_</a> + 1, hana::nothing) == <span class="charliteral">&#39;x&#39;</span>, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    424. 

  424. </div>
    425. 

  425. </div>
    426. 

  426. <a id="aa4c8c0b1ef3d8206974734b93646c9fd"></a>
    427. 

  427. <h2 class="memtitle"><span class="permalink"><a href="#aa4c8c0b1ef3d8206974734b93646c9fd">&#9670;&nbsp;</a></span>sfinae</h2>
    428. 

  428. 

  429. <div class="memitem">
    430. 

  430. <div class="memproto">
    431. 

  431. <div class="memtemplate">
    432. 

  432. template&lt;typename ... T&gt; </div>
    433. 

  433. <table class="mlabels">
    434. 

  434.   <tr>
    435. 

  435.   <td class="mlabels-left">
    436. 

  436.       <table class="memname">
    437. 

  437.         <tr>
    438. 

  438.           <td class="memname">auto sfinae</td>
    439. 

  439.         </tr>
    440. 

  440.       </table>
    441. 

  441.   </td>
    442. 

  442.   <td class="mlabels-right">
    443. 

  443. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
    444. 

  444.   </tr>
    445. 

  445. </table>
    446. 

  446. </div><div class="memdoc">
    447. 

  447. <b>Initial value:</b><div class="fragment"><div class="line">= [](<span class="keyword">auto</span>&amp;&amp; f) {</div><div class="line">        <span class="keywordflow">return</span> [perfect-<a class="code" href="group__group-functional.html#ga41ada6b336e9d5bcb101ff0c737acbd0">capture</a>](<span class="keyword">auto</span>&amp;&amp; ...x) {</div><div class="line">            <span class="keywordflow">if</span> (decltype(forwarded(f)(forwarded(x)...)) is well-formed)</div><div class="line">                <span class="keywordflow">return</span> <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(forwarded(f)(forwarded(x)...));</div><div class="line">            <span class="keywordflow">else</span></div><div class="line">                <span class="keywordflow">return</span> <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>;</div><div class="line">        };</div><div class="line">    }</div></div><!-- fragment -->
    448. 

  448. <p>Calls a function if the call expression is well-formed. </p>
    449. 

  449. <p>Given a function <code>f</code>, <code>sfinae</code> returns a new function applying <code>f</code> to its arguments and returning <code>just</code> the result if the call is well-formed, and <code>nothing</code> otherwise. In other words, <code>sfinae(f)(x...)</code> is <code>just(f(x...))</code> if that expression is well-formed, and <code>nothing</code> otherwise. Note, however, that it is possible for an expression <code>f(x...)</code> to be well-formed as far as SFINAE is concerned, but trying to actually compile <code>f(x...)</code> still fails. In this case, <code>sfinae</code> won't be able to detect it and a hard failure is likely to happen.</p>
    450. 

  450. <dl class="section note"><dt>Note</dt><dd>The function given to <code>sfinae</code> must not return <code>void</code>, since <code>just(void)</code> does not make sense. A compilation error is triggered if the function returns void.</dd></dl>
    451. 

  451. <h2>Example </h2>
    452. 

  452. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="config_8hpp.html">boost/hana/config.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    BOOST_HANA_CONSTEXPR_LAMBDA <span class="keyword">auto</span> incr = [](<span class="keyword">auto</span> x) -&gt; decltype(x + 1) {</div><div class="line">        <span class="keywordflow">return</span> x + 1;</div><div class="line">    };</div><div class="line"></div><div class="line">    <a class="code" href="group__group-assertions.html#gac7aafc41e4dcc7d1f1929fb00f010d2a">BOOST_HANA_CONSTEXPR_CHECK</a>(hana::sfinae(incr)(1) == hana::just(2));</div><div class="line"></div><div class="line">    <span class="keyword">struct </span>invalid { };</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(hana::sfinae(incr)(invalid{}) == hana::nothing);</div><div class="line">}</div></div><!-- fragment --> 
    453. 

  453. </div>
    454. 

  454. </div>
    455. 

  455. <a id="a1047a69893a62cd36eb0d2934f85e0e7"></a>
    456. 

  456. <h2 class="memtitle"><span class="permalink"><a href="#a1047a69893a62cd36eb0d2934f85e0e7">&#9670;&nbsp;</a></span>is_just</h2>
    457. 

  457. 

  458. <div class="memitem">
    459. 

  459. <div class="memproto">
    460. 

  460. <div class="memtemplate">
    461. 

  461. template&lt;typename ... T&gt; </div>
    462. 

  462. <table class="mlabels">
    463. 

  463.   <tr>
    464. 

  464.   <td class="mlabels-left">
    465. 

  465.       <table class="memname">
    466. 

  466.         <tr>
    467. 

  467.           <td class="memname">constexpr auto is_just</td>
    468. 

  468.         </tr>
    469. 

  469.       </table>
    470. 

  470.   </td>
    471. 

  471.   <td class="mlabels-right">
    472. 

  472. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
    473. 

  473.   </tr>
    474. 

  474. </table>
    475. 

  475. </div><div class="memdoc">
    476. 

  476. <b>Initial value:</b><div class="fragment"><div class="line">= [](<span class="keyword">auto</span> <span class="keyword">const</span>&amp; m) {</div><div class="line">        <span class="keywordflow">return</span> m is a <a class="code" href="structboost_1_1hana_1_1optional.html#a8ad818d6df7379b0adefa45d2ac844d0">just</a>(x);</div><div class="line">    }</div></div><!-- fragment -->
    477. 

  477. <p>Return whether an <code>optional</code> contains a value. </p>
    478. 

  478. <p>Specifically, returns a compile-time true-valued <code>Logical</code> if <code>m</code> is of the form <code>just(x)</code> for some <code>x</code>, and a false-valued one otherwise.</p>
    479. 

  479. <h2>Example </h2>
    480. 

  480. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="not_8hpp.html">boost/hana/not.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>( hana::is_just(hana::just(<span class="charliteral">&#39;x&#39;</span>)));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>( hana::is_just(hana::just(hana::nothing)));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(!hana::is_just(hana::nothing));</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    481. 

  481. </div>
    482. 

  482. </div>
    483. 

  483. <a id="a4eb266d123883c23853f7731a6c5953f"></a>
    484. 

  484. <h2 class="memtitle"><span class="permalink"><a href="#a4eb266d123883c23853f7731a6c5953f">&#9670;&nbsp;</a></span>is_nothing</h2>
    485. 

  485. 

  486. <div class="memitem">
    487. 

  487. <div class="memproto">
    488. 

  488. <div class="memtemplate">
    489. 

  489. template&lt;typename ... T&gt; </div>
    490. 

  490. <table class="mlabels">
    491. 

  491.   <tr>
    492. 

  492.   <td class="mlabels-left">
    493. 

  493.       <table class="memname">
    494. 

  494.         <tr>
    495. 

  495.           <td class="memname">constexpr auto is_nothing</td>
    496. 

  496.         </tr>
    497. 

  497.       </table>
    498. 

  498.   </td>
    499. 

  499.   <td class="mlabels-right">
    500. 

  500. <span class="mlabels"><span class="mlabel">related</span></span>  </td>
    501. 

  501.   </tr>
    502. 

  502. </table>
    503. 

  503. </div><div class="memdoc">
    504. 

  504. <b>Initial value:</b><div class="fragment"><div class="line">= [](<span class="keyword">auto</span> <span class="keyword">const</span>&amp; m) {</div><div class="line">        <span class="keywordflow">return</span> m is a <a class="code" href="structboost_1_1hana_1_1optional.html#a0c315287473aa87fe5ccd9d8a1c10186">nothing</a>;</div><div class="line">    }</div></div><!-- fragment -->
    505. 

  505. <p>Return whether an <code>optional</code> is empty. </p>
    506. 

  506. <p>Specifically, returns a compile-time true-valued <code>Logical</code> if <code>m</code> is a <code>nothing</code>, and a false-valued one otherwise.</p>
    507. 

  507. <h2>Example </h2>
    508. 

  508. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="not_8hpp.html">boost/hana/not.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>( hana::is_nothing(hana::nothing));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(!hana::is_nothing(hana::just(<span class="charliteral">&#39;x&#39;</span>)));</div><div class="line"><a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(!hana::is_nothing(hana::just(hana::nothing)));</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    509. 

  509. </div>
    510. 

  510. </div>
    511. 

  511. <h2 class="groupheader">Member Function Documentation</h2>
    512. 

  512. <a id="a6eeef1512294d408857805eb3a790230"></a>
    513. 

  513. <h2 class="memtitle"><span class="permalink"><a href="#a6eeef1512294d408857805eb3a790230">&#9670;&nbsp;</a></span>operator-&gt;()</h2>
    514. 

  514. 

  515. <div class="memitem">
    516. 

  516. <div class="memproto">
    517. 

  517. <div class="memtemplate">
    518. 

  518. template&lt;typename ... T&gt; </div>
    519. 

  519.       <table class="memname">
    520. 

  520.         <tr>
    521. 

  521.           <td class="memname">constexpr T* <a class="el" href="structboost_1_1hana_1_1optional.html">boost::hana::optional</a>&lt; T &gt;::operator-&gt; </td>
    522. 

  522.           <td>(</td>
    523. 

  523.           <td class="paramname"></td><td>)</td>
    524. 

  524.           <td></td>
    525. 

  525.         </tr>
    526. 

  526.       </table>
    527. 

  527. </div><div class="memdoc">
    528. 

  528. 

  529. <p>Returns a pointer to the contained value, or a <code>nullptr</code> if the <code>optional</code> is empty. </p>
    530. 

  530. <dl class="section note"><dt>Note</dt><dd>Overloads of this method are provided for both the <code>const</code> and the non-<code>const</code> cases.</dd></dl>
    531. 

  531. <h2>Example </h2>
    532. 

  532. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="type_8hpp.html">boost/hana/type.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;string&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    static_assert(hana::just(<span class="charliteral">&#39;x&#39;</span>).<a class="code" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58">value</a>() == <span class="charliteral">&#39;x&#39;</span>, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(*hana::just(hana::type_c&lt;int&gt;) == hana::type_c&lt;int&gt;);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga4796ae107d58b67e0bbccd5ae6f70101">BOOST_HANA_RUNTIME_CHECK</a>(hana::just(std::string{<span class="stringliteral">&quot;abcd&quot;</span>})-&gt;<a class="code" href="group__group-Foldable.html#ga8ec3ac9a6f5014db943f61ebc9e1e36e">size</a>() == 4);</div><div class="line"></div><div class="line">    <span class="comment">// hana::nothing.value(); // compile-time error</span></div><div class="line">}</div></div><!-- fragment --> 
    533. 

  533. </div>
    534. 

  534. </div>
    535. 

  535. <a id="acdcf71b43d4e27d5efbf0ad875d0be58"></a>
    536. 

  536. <h2 class="memtitle"><span class="permalink"><a href="#acdcf71b43d4e27d5efbf0ad875d0be58">&#9670;&nbsp;</a></span>value()</h2>
    537. 

  537. 

  538. <div class="memitem">
    539. 

  539. <div class="memproto">
    540. 

  540. <div class="memtemplate">
    541. 

  541. template&lt;typename ... T&gt; </div>
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  542.       <table class="memname">
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  543.         <tr>
    544. 

  544.           <td class="memname">constexpr T&amp; <a class="el" href="structboost_1_1hana_1_1optional.html">boost::hana::optional</a>&lt; T &gt;::value </td>
    545. 

  545.           <td>(</td>
    546. 

  546.           <td class="paramname"></td><td>)</td>
    547. 

  547.           <td></td>
    548. 

  548.         </tr>
    549. 

  549.       </table>
    550. 

  550. </div><div class="memdoc">
    551. 

  551. 

  552. <p>Extract the content of an <code>optional</code>, or fail at compile-time. </p>
    553. 

  553. <p>If <code>*this</code> contains a value, that value is returned. Otherwise, a static assertion is triggered.</p>
    554. 

  554. <dl class="section note"><dt>Note</dt><dd>Overloads of this method are provided for the cases where <code>*this</code> is a reference, a rvalue-reference and their <code>const</code> counterparts.</dd></dl>
    555. 

  555. <h2>Example </h2>
    556. 

  556. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="type_8hpp.html">boost/hana/type.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;string&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    static_assert(hana::just(<span class="charliteral">&#39;x&#39;</span>).<a class="code" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58">value</a>() == <span class="charliteral">&#39;x&#39;</span>, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(*hana::just(hana::type_c&lt;int&gt;) == hana::type_c&lt;int&gt;);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga4796ae107d58b67e0bbccd5ae6f70101">BOOST_HANA_RUNTIME_CHECK</a>(hana::just(std::string{<span class="stringliteral">&quot;abcd&quot;</span>})-&gt;<a class="code" href="group__group-Foldable.html#ga8ec3ac9a6f5014db943f61ebc9e1e36e">size</a>() == 4);</div><div class="line"></div><div class="line">    <span class="comment">// hana::nothing.value(); // compile-time error</span></div><div class="line">}</div></div><!-- fragment --> 
    557. 

  557. </div>
    558. 

  558. </div>
    559. 

  559. <a id="a237b2147dd9b72bb2dfa4a268056e4c2"></a>
    560. 

  560. <h2 class="memtitle"><span class="permalink"><a href="#a237b2147dd9b72bb2dfa4a268056e4c2">&#9670;&nbsp;</a></span>operator *()</h2>
    561. 

  561. 

  562. <div class="memitem">
    563. 

  563. <div class="memproto">
    564. 

  564. <div class="memtemplate">
    565. 

  565. template&lt;typename ... T&gt; </div>
    566. 

  566.       <table class="memname">
    567. 

  567.         <tr>
    568. 

  568.           <td class="memname">constexpr T&amp; <a class="el" href="structboost_1_1hana_1_1optional.html">boost::hana::optional</a>&lt; T &gt;::operator * </td>
    569. 

  569.           <td>(</td>
    570. 

  570.           <td class="paramname"></td><td>)</td>
    571. 

  571.           <td></td>
    572. 

  572.         </tr>
    573. 

  573.       </table>
    574. 

  574. </div><div class="memdoc">
    575. 

  575. 

  576. <p>Equivalent to <code><a class="el" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58" title="Extract the content of an optional, or fail at compile-time.">value()</a></code>, provided for convenience. </p>
    577. 

  577. <dl class="section note"><dt>Note</dt><dd>Overloads of this method are provided for the cases where <code>*this</code> is a reference, a rvalue-reference and their <code>const</code> counterparts.</dd></dl>
    578. 

  578. <h2>Example </h2>
    579. 

  579. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="equal_8hpp.html">boost/hana/equal.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="type_8hpp.html">boost/hana/type.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;string&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    static_assert(hana::just(<span class="charliteral">&#39;x&#39;</span>).<a class="code" href="structboost_1_1hana_1_1optional.html#acdcf71b43d4e27d5efbf0ad875d0be58">value</a>() == <span class="charliteral">&#39;x&#39;</span>, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga2c5006540936d9f8880e3a39f4fcc035">BOOST_HANA_CONSTANT_CHECK</a>(*hana::just(hana::type_c&lt;int&gt;) == hana::type_c&lt;int&gt;);</div><div class="line">    <a class="code" href="group__group-assertions.html#ga4796ae107d58b67e0bbccd5ae6f70101">BOOST_HANA_RUNTIME_CHECK</a>(hana::just(std::string{<span class="stringliteral">&quot;abcd&quot;</span>})-&gt;<a class="code" href="group__group-Foldable.html#ga8ec3ac9a6f5014db943f61ebc9e1e36e">size</a>() == 4);</div><div class="line"></div><div class="line">    <span class="comment">// hana::nothing.value(); // compile-time error</span></div><div class="line">}</div></div><!-- fragment --> 
    580. 

  580. </div>
    581. 

  581. </div>
    582. 

  582. <a id="a1f511267d94c178e4ef955da60f0888d"></a>
    583. 

  583. <h2 class="memtitle"><span class="permalink"><a href="#a1f511267d94c178e4ef955da60f0888d">&#9670;&nbsp;</a></span>value_or()</h2>
    584. 

  584. 

  585. <div class="memitem">
    586. 

  586. <div class="memproto">
    587. 

  587. <div class="memtemplate">
    588. 

  588. template&lt;typename ... T&gt; </div>
    589. 

  589. <div class="memtemplate">
    590. 

  590. template&lt;typename U &gt; </div>
    591. 

  591.       <table class="memname">
    592. 

  592.         <tr>
    593. 

  593.           <td class="memname">decltype(auto) constexpr <a class="el" href="structboost_1_1hana_1_1optional.html">boost::hana::optional</a>&lt; T &gt;::value_or </td>
    594. 

  594.           <td>(</td>
    595. 

  595.           <td class="paramtype">U &amp;&amp;&#160;</td>
    596. 

  596.           <td class="paramname"><em>default_</em></td><td>)</td>
    597. 

  597.           <td></td>
    598. 

  598.         </tr>
    599. 

  599.       </table>
    600. 

  600. </div><div class="memdoc">
    601. 

  601. 

  602. <p>Return the contents of an <code>optional</code>, with a fallback result. </p>
    603. 

  603. <p>If <code>*this</code> contains a value, that value is returned. Otherwise, the default value provided is returned.</p>
    604. 

  604. <dl class="section note"><dt>Note</dt><dd>Overloads of this method are provided for the cases where <code>*this</code> is a reference, a rvalue-reference and their <code>const</code> counterparts.</dd></dl>
    605. 

  605. <dl class="params"><dt>Parameters</dt><dd>
    606. 

  606.   <table class="params">
    607. 

  607.     <tr><td class="paramname"><a class="el" href="structboost_1_1hana_1_1default__.html" title="Mark a tag-dispatched method implementation as a default implementation.">default_</a></td><td>The default value to return if <code>*this</code> does not contain a value.</td></tr>
    608. 

  608.   </table>
    609. 

  609.   </dd>
    610. 

  610. </dl>
    611. 

  611. <h2>Example </h2>
    612. 

  612. <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="optional_8hpp.html">boost/hana/optional.hpp</a>&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line">static_assert(hana::just(1).<a class="code" href="structboost_1_1hana_1_1optional.html#a1f511267d94c178e4ef955da60f0888d">value_or</a>(<span class="charliteral">&#39;x&#39;</span>) == 1, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(hana::nothing.<a class="code" href="structboost_1_1hana_1_1optional.html#a1f511267d94c178e4ef955da60f0888d">value_or</a>(<span class="charliteral">&#39;x&#39;</span>) == <span class="charliteral">&#39;x&#39;</span>, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() { }</div></div><!-- fragment --> 
    613. 

  613. </div>
    614. 

  614. </div>
    615. 

  615. </div><!-- contents -->
    616. 

  616. </div><!-- doc-content -->
    617. 

  617. <!--
    618. 

  618. Copyright Louis Dionne 2013-2017
    619. 

  619. Distributed under the Boost Software License, Version 1.0.
    620. 

  620. (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
    621. 

  621. -->
    622. 

  622. <!-- boost-no-inspect -->
    623. 

  623. <!-- HTML footer for doxygen 1.8.9.1-->
    624. 

  624. <!-- start footer part -->
    625. 

  625. <div id="nav-path" class="navpath"><!-- id is needed for treeview function! -->
    626. 

  626.   <ul>
    627. 

  627.     <li class="navelem"><b>boost</b></li><li class="navelem"><a class="el" href="namespaceboost_1_1hana.html">hana</a></li><li class="navelem"><a class="el" href="structboost_1_1hana_1_1optional.html">optional</a></li>
    628. 

  628.   </ul>
    629. 

  629. </div>
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  630. </body>
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  631. </html>
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